Today, the students hear about potential projects in Brazil, Zambia, Tanzania, and India. Last week, they learned about communities in Guatemala, Ghana, Lesotho, China, and Honduras, where Smith's idea for using toilet tank technology to ensure a steady supply of safe, chlorinated water has been embraced and improved by local plumbers; it's already spread to dozens of water treatment systems. Though Smith's MIT uniform consists of jeans and sweaters, slides from her trips show her in her traveling wardrobe. In the villages, she often wears what she calls her "development dresses" as a sign of respect for community leaders.

Smith tries to keep her life as simple as her inventions. She owns no car, commutes by train, and takes a break from work on weekends to paddle her inflatable kayak. Her growing program budget rests on the shaky foundation of grant money, annual fund-raising, and the faith that both will continue. "Everything we do is on a shoestring," she says. "I like operating that way."

Promoted to senior lecturer in mechanical engineering this year, Smith has reached the top of the nontenured ladder and professes no further academic ambitions. With her increasing administrative responsibilities, she now sometimes spends whole days in meetings instead of brainstorming simpler technologies with students. "I still want to be a teacher," she says. "I want to be an inventor. My favorite thing in the world is to go to villages and talk about the issues they have."

When the world's problems seem too overwhelming, Smith recalls the words of activist Catholic priest Daniel ­Berrigan: "One cannot level one's moral lance at every evil in the universe. There are just too many of them. But you can do something, and the difference between doing something and doing nothing is everything."